Because of being excellent in toughness and flame retardancy, cellulose acylate films have been employed in photographic supports and various optical materials. In recent years, in particular, cellulose acylate films are frequently employed as optical transparent films for liquid crystal display units. Owing to the high optical transparency and high optical isotropy, cellulose acylate films are favorable as optical materials for units with the use of polarization such as liquid crystal display units. Therefore, cellulose acylate films have been employed as optical compensation film supports whereby display in looking from an angle can compensated (viewing angle compensation).
A polarizing sheet, which is one of members constituting a liquid crystal display unit, is constructed by bonding a polarizer-protecting film to at least one side of a polarizer. In general, a polarizer is obtained by dyeing a stretched polyvinyl alcohol (PVA)-base film with iodine or a dichromatic dye. As the polarizer-protecting film, cellulose acylate films, in particular, triacetyl cellulose films which can be bonded directly to PVA are employed in may cases. Such a polarizer-protecting film should be excellent in optical isotropy and the characteristics of a polarizing plate largely depend on the optical characteristics of the polarizer-protecting film.
In liquid crystal display unit in these days, it is more strongly required to improve viewing angle characteristics. Thus, optical transparent films to be used as a polarizer-protecting film, an optical compensation film support, etc. should be optically isotropic. To be optically isotropic, it is important to have a small retardation value represented by the product of the birefringence and thickness of an optical film. To improve the display in looking from an angle, it is particularly needed to lower not only the retardation value (Re) but also the retardation value in the film thickness direction (Rth). More specifically speaking, it is needed that, in the case of evaluating the optical properties of an optical transparent film, Re measured in plane is a small value and Re shows no change even though the measurement angle is varied.
For solving this problem, it has been urgently required to further improve a cellulose acylate film, which has favorable properties in boding to PVA, by lowering its optical anisotropy. More specifically speaking, a preferable optical transparent film being optically isotropic is a cellulose acylate film that has a retardation value Re of almost zero and shows little change in retardation angle (i.e., Rth being almost zero too).
In producing a cellulose acylate film, it has been a practice to add a compound called a plasticizer to thereby improve the film-forming performance. Examples of the plasticizer include phosphoric acid triesters such as triphenyl phosphate and biphenyldiphenyl phosphate and phthalic acid esters (see, for example, Purasuchikku Zairyo Koza, vol. 17, Nikkan Kogyo Shinbun, Ltd., Senisokei Jushi, p. 121 (1970)). It is known that some of these plasticizers have an effect of lowering the optical anisotropy of a cellulose acylate film. For example, specific fatty acid esters are disclosed (see, for example, JP-A-2001-247717). However, these known compounds can only insufficiently lower the optical anisotropy of a cellulose acylate film.
As a method of producing a biaxial optical compensation film to be used in liquid crystal display units in recent years, there has been proposed a method which comprises spreading a liquefied solid polymer dissolved in a solvent (vehicle) on a supporting material, drying the same, subjecting a transparent film (nx=ny or nx≅ny) made of the thus solidified matter to either a stretching treatment or a shrinkage treatment or both thereof, and thereby orientating molecules in the plane to impart the characteristics nx>ny>nz. In this method, it is insufficient to merely lower the optical anisotropy as discussed above. That is to say, there has been required a film showing no optical anisotropy even after the stretching treatment or the shrinkage treatment (see JP-A-2003-315541, JP-A-2003-344856, JP-A-2004-46097 and JP-A-2004-78203).